Parasites appear frequently in biology-inspired fiction from ancient times onwards, with a flowering in the nineteenth century. These include intentionally disgusting alien monsters in science fiction films, often with analogues in nature. Authors and scriptwriters have, to some extent, exploited parasite biology: lifestyles including parasitoid, behaviour-altering parasite, brood parasite, parasitic castrator, and many forms of vampire are found in books and films. Some fictional parasites, like Count Dracula and Alien's Xenomorphs, have become well known in their own right.

In evolutionary biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". According to the immunologist John Playfair, the term 'parasite' is distinctly derogatory in common usage, where a parasite is "a sponger, a lazy profiteer, a drain on society". The idea is however much older. In ancient Rome, the parasitus was an accepted role in Roman society, in which a person could live off the hospitality of others, in return for "flattery, simple services, and a willingness to endure humiliation".

Parasitism featured repeatedly as a literary motif in the nineteenth century, though the mechanisms, biological or otherwise, are not always described in detail. For example, the eponymous Beetle in The Beetle by Richard Marsh, 1897, is parasitic and symbolically castrates the human protagonist. Bram Stoker's 1897 Dracula starts out as an apparently human host, welcoming guests to his home, before revealing his parasitic vampire nature. Conan Doyle's Parasite, in his 1894 book The Parasite, makes use of a form of mind control similar to the mesmerism of the Victorian era; it works on some hosts but not others.

Parasites, represented as extraterrestrial aliens or unnatural beings, are seen in science fiction as distasteful, in contrast to (mutualistic) symbiosis, and sometimes horrible. Practical uses can be made of them, but humans who do so may be destroyed by them. For example, Mira Grant's 2013 novel Parasite envisages a world where people's immune systems are maintained by genetically engineered tapeworms. They form readily understood characters, since, as Gary Westfahl explains, parasites need to exploit their hosts to survive and reproduce.

The social anthropologist Marika Moisseeff argues that Hollywood science fiction favours insects as villain characters because of their parasitism and their swarming behaviour. Such films, she continues, depict a ceaseless war of culture and nature as involving extraterrestrial species somewhat resembling insects, with humans as their hosts."

The range of accounts of fictional parasites and the media used to describe them have greatly increased since the nineteenth century, spanning among other things literary novels, science fiction novels and films, horror films, and video games. The table illustrates the variety of themes and approaches that have become possible.

Kyle Munkittrick, on the Discover magazine website, writes that the great majority of aliens, far from being as strange as possible, are humanoid. Ben Guarino, in The Washington Post, observes that despite all the "cinematic aliens' gravid grotesquerie", earthly parasites have more horrible ways of life. Guarino cites parasitic wasps that lay their eggs inside living caterpillars, inspiring A. E. Van Vogt's 1939 story "Discord in Scarlet", Robert Heinlein's 1951 novel The Puppet Masters, and Ridley Scott's 1979 film Alien. The eponymous Alien has a "dramatic" life-cycle. Giant eggs hatch into face-huggers that grasp the host's mouth, forcing him to swallow an embryo. It rapidly grows in his intestines, soon afterwards erupting from his chest and growing into a gigantic predatory animal resembling an insect. Guarino cites the parasitologist Michael J. Smout as saying that the "massive changes" are feasible, giving the example of flatworms that transform from an egg to a tadpole-like form to an infective worm. The biologist Claude dePamphilis agrees, too, that parasites can acquire genes from their hosts, giving as example a broomrape plant that had taken up genes from its host on 52 occasions, having thoroughly overcome the host plant's defences. They suggest further themes for future science fiction films, including emerald jewel wasps that turn cockroaches into subservient puppets, able to crawl but unable to act independently; or the barnacle-like crustaceans that castrate their crab hosts, or grow into their brains, altering their behaviour to care for the young barnacles. All the same, a 2013 poll of scientists and engineers by Popular Mechanics magazine revealed that the parasite-based science fiction films The War of the Worlds (Byron Haskin, 1953) and Alien were among their top ten favourites.

Several types of parasite, corresponding more or less accurately to some of those known in biology, are found in literature. These include haematophagic parasites (fictional vampires), parasitoids, behaviour-altering parasites, brood parasites, parasitic castrators, and trophically transmitted parasites, as detailed below.